Given a command in SQL;
SELECT ...
FROM ...
GROUP BY ...
Can I group by something that isn't in the SELECT line?
Yes.
This is often used in the superaggregate queries like this:
SELECT AVG(cnt)
FROM (
SELECT COUNT(*) AS cnt
FROM sales
GROUP BY
product
HAVING COUNT(*) > 10
) q
, which aggregate the aggregates.
Yes of course e.g.
select
count(*)
from
some_table_with_updated_column
group by
trunc(updated, 'MM.YYYY')
Yes you can do it, but if you do that you won't be able to tell which result is for which group.
As a result, you almost always want to return the columns you've grouped by in the select clause. But you don't have to.
Yes, you can. Example:
select count(1)
from sales
group by salesman_id
What you can't do, of course, if having something on your select clause (other than aggregate functions) that are not part of the group by clause.
Hmm, I think the question should have been in the other way round like,
Can I SELECT something that is not there in the GROUP BY?
It's alright to write a code like:
SELECT customerId, count(orderId) FROM orders
GROUP BY customerId, orderedOn
If you want to find out the number of orders done by a customer datewise.
But you cannot do it the other way round:
SELECT customerId, orderedOn count(orderId) FROM orders
GROUP BY customerId
You can issue an aggregate function on the column that is not there in the group by. But you cannot give it in the select line without the aggregate function. As it will not make much sense. Like for the above query. You group by just customerId for order counts and you want the date also to be printed in the output??!! You don't involve the date factor in the group for counting then will it mean something to have a date in it?
I don't know about other DBMS' but DB2/z, for one, does this just fine. It's not required to have the column in the select portion but, of course, it does have to extract the data from the table in order to aggregate so you're probably not saving any time by leaving it off. You should only select the columns that you need, aggregation of the data is a separate task from that.
I'm pretty certain the SQL standard allows this (although that's only based on the knowledge that the mainframe DB2 product follows it pretty closely).
Related
I want to list out the product which has highest sales amount on date wise.
note: highest sales amount in the sense max(sum(sales_amnt)...
by using if or case In the procedure in sap hana SQL....
I did this by using with the clause :
/--------------------------CORRECT ONE ----------------------------------------------/
WITH ranked AS
(
SELECT Dense_RAnk() OVER (ORDER BY SUM("SALES_AMNT"), "SALES_DATE", "PROD_NAME") as rank,
SUM("SALES_AMNT") AS Amount, "PROD_NAME",count(*), "SALES_DATE" FROM "KABIL"."DATE"
GROUP BY "SALES_DATE", "PROD_NAME"
)
SELECT "SALES_DATE", "PROD_NAME",Amount
FROM ranked
WHERE rank IN ( select MAX(rank) from ranked group by "SALES_DATE")
ORDER BY "SALES_DATE" DESC;
this is my table
You can not use IF along with SELECT statement. Note that, you can achieve most of boolean logics with CASE statement syntax
In select, you are applying it over a column and your logic will be executed as many as times the count of result set rows. Hence , righting an imperative logic is not well appreciated. Still, if you want to do the same, create a calculation view and use intermediate calculated columns to achieve what you are expecting .
try this... i got an answer ...
select "SALES_DATE","PROD_NAME",sum("SALES_AMNT")
from "KABIL"."DATE"
group by "SALES_DATE","PROD_NAME"
having (SUM("SALES_AMNT"),"SALES_DATE") IN (select
MAX(SUM_SALES),"SALES_DATE"
from (select SUM("SALES_AMNT")
as
SUM_SALES,"SALES_DATE","PROD_NAME"
from "KABIL"."DATE"
group by "SALES_DATE","PROD_NAME"
)
group by "SALES_DATE");
I'm trying to use a select query to show all the customers in my database who made more than 5 complaints in the past. I tried this query:
SELECT customer_ID, COUNT(customer_feedback.feedback_type) AS complaints
FROM customer_feedback
WHERE complaints>5
GROUP BY customer_ID;
But it doesn't work. Access does not recognize the expression 'complaints' in the WHERE clause. So I tried this, which was even worse:
SELECT customer_ID, COUNT(customer_feedback.feedback_type) AS complaints
FROM customer_feedback
WHERE COUNT(customer_feedback.feedback_type)>1
GROUP BY customer_ID;
I'm sure there's a simple solution that I just can't think of right now.
SELECT customer_ID, COUNT(customer_feedback.feedback_type) AS complaints
FROM customer_feedback
GROUP BY customer_ID
HAVING COUNT(customer_feedback.feedback_type)>5;
In SQL Server, this would be a "having" clause part of the group by
select something, count(*)
from somewhere
group by something
having count(*) > 5
I need a bit of help straightening out something, I know it's a very easy easy question but it's something that is slightly confusing me in SQL.
This SQL query throws a 'not a GROUP BY expression' error in Oracle. I understand why, as I know that once I group by an attribute of a tuple, I can no longer access any other attribute.
SELECT *
FROM order_details
GROUP BY order_no
However this one does work
SELECT SUM(order_price)
FROM order_details
GROUP BY order_no
Just to concrete my understanding on this.... Assuming that there are multiple tuples in order_details for each order that is made, once I group the tuples according to order_no, I can still access the order_price attribute for each individual tuple in the group, but only using an aggregate function?
In other words, aggregate functions when used in the SELECT clause are able to drill down into the group to see the 'hidden' attributes, where simply using 'SELECT order_no' will throw an error?
In standard SQL (but not MySQL), when you use GROUP BY, you must list all the result columns that are not aggregates in the GROUP BY clause. So, if order_details has 6 columns, then you must list all 6 columns (by name - you can't use * in the GROUP BY or ORDER BY clauses) in the GROUP BY clause.
You can also do:
SELECT order_no, SUM(order_price)
FROM order_details
GROUP BY order_no;
That will work because all the non-aggregate columns are listed in the GROUP BY clause.
You could do something like:
SELECT order_no, order_price, MAX(order_item)
FROM order_details
GROUP BY order_no, order_price;
This query isn't really meaningful (or most probably isn't meaningful), but it will 'work'. It will list each separate order number and order price combination, and will give the maximum order item (number) associated with that price. If all the items in an order have distinct prices, you'll end up with groups of one row each. OTOH, if there are several items in the order at the same price (say £0.99 each), then it will group those together and return the maximum order item number at that price. (I'm assuming the table has a primary key on (order_no, order_item) where the first item in the order has order_item = 1, the second item is 2, etc.)
The order in which SQL is written is not the same order it is executed.
Normally, you would write SQL like this:
SELECT
FROM
JOIN
WHERE
GROUP BY
HAVING
ORDER BY
Under the hood, SQL is executed like this:
FROM
JOIN
WHERE
GROUP BY
HAVING
SELECT
ORDER BY
Reason why you need to put all the non-aggregate columns in SELECT to the GROUP BY is the top-down behaviour in programming. You cannot call something you have not declared yet.
Read more: https://sqlbolt.com/lesson/select_queries_order_of_execution
SELECT *
FROM order_details
GROUP BY order_no
In the above query you are selecting all the columns because of that its throwing an error not group by something like..
to avoid that you have to mention all the columns whichever in select statement all columns must be in group by clause..
SELECT *
FROM order_details
GROUP BY order_no,order_details,etc
etc it means all the columns from order_details table.
To use group by clause you have to mention all the columns from select statement in to group by clause but not the column from aggregate function.
TO do this instead of group by you can use partition by clause you can use only one port to group as a partition by.
you can also make it as partition by 1
use Common table expression(CTE) to avoid this issue.
multiple CTes also come handy, pasting a case where I have used...maybe helpful
with ranked_cte1 as
( select r.mov_id,DENSE_RANK() over ( order by r.rev_stars desc )as rankked from ratings r ),
ranked_cte2 as ( select * from movie where mov_id=(select mov_id from ranked_cte1 where rankked=7 ) ) select * from ranked_cte2
select * from movie where mov_id=902
i tried using this query:
"SELECT * FROM guests WHERE event_id=".$id." GROUP BY member_id;"
and I'm getting this error:
ERROR: column "guests.id" must appear in the GROUP BY clause or be used in an aggregate function
can anyone explain how i can work around this?
You can't Group By without letting the Select know what to take, and how to group.
Try
SELECT guests.member_id FROM guests WHERE event_id=".$id." GROUP BY member_id;
IF you need to get more info from this table about the guests, you'll need to add it to the Group By.
Plus, it seems like your select should actually be
SELECT guests.id FROM guests WHERE event_id=".$id." GROUP BY id;
Each of the columns used in a group by query needs to be specifically called out (ie, don't do SELECT * FROM ...), as you need to use them in some sort of aggregate function (min/max/sum/avg/count/etc) or be part of the group by clause.
For example:
SELECT instrument, detector, min(date_obs), max(date_obs)
FROM observations
WHERE observatory='SOHO'
GROUP BY instrument, detector;
According to tips from MySQL performance wiki:
Don't use DISTINCT when you have or could use GROUP BY.
Can somebody post example of queries where GROUP BY can be used instead of DISTINCT?
If you know that two columns from your result are always directly related then it's slower to do this:
SELECT DISTINCT CustomerId, CustomerName FROM (...)
than this:
SELECT CustomerId, CustomerName FROM (...) GROUP BY CustomerId
because in the second case it only has to compare the id, but in the first case it has to compare both fields. This is a MySQL specific trick. It won't work with other databases.
SELECT Code
FROM YourTable
GROUP BY Code
vs
SELECT DISTINCT Code
FROM YourTable
The basic rule : Put all the columns from the SELECT clause into the GROUP BY clause
so
SELECT DISTINCT a,b,c FROM D
becomes
SELECT a,b,c FROM D GROUP BY a,b,c
Example.
Relation customer(ssnum,name, zipcode, address) PK(ssnum). ssnum is social security number.
SQL:
Select DISTINCT ssnum from customer where zipcode=1234 group by name
This SQL statement returns unique records for those customer's that have zipcode 1234. At the end results are grouped by name.
Here DISTINCT is no not necessary. because you are selecting ssnum which is already unique because ssnun is primary key. two person can not have same ssnum.
In this case Select ssnum from customer where zipcode=1234 group by name will give better performance than "... DISTINCT.......".
DISTINCT is an expensive operation in a DBMS.